An image sensor is a semiconductor device for converting an optical image into an electrical signal. The image sensor is roughly classified as a charge coupled device (CCD) image sensor or a complementary metal oxide semiconductor (CMOS) image sensor (CIS).
In a related art, a photodiode is formed in a substrate with transistor circuitry using ion implantation. As the size of a photodiode reduces more and more for the purpose of increasing the number of pixels without an increase in a chip size, the area of a light receiving portion reduces, so that image quality reduces.
Also, since a stack height does not reduce as much as the reduction in the area of the light receiving portion, the number of photons incident to the light receiving portion also reduces due to diffraction of light, called airy disk.
As an alternative to overcome this limitation, an attempt of forming a photodiode using amorphous silicon (Si), or forming a readout circuitry in a Si substrate and forming a photodiode on the readout circuitry using a method such as wafer-to-wafer bonding has been made (referred to as a “three-dimensional (3D) image sensor). The photodiode is connected with the readout circuitry through an interconnection.
Meanwhile, according to a related art, since the both the source and the drain of a transfer transistor of the readout circuitry are heavily doped with N-type impurities, a charge sharing phenomenon occurs. When the charge sharing phenomenon occurs, the sensitivity of an output image is reduced and an image error may be generated.
Also, according to the related art, because a photo charge does not readily move between the photodiode and the readout circuitry, a dark current is generated or saturation and sensitivity reduce.